1. Field of the Invention
The present invention relates to devices and method of thermally insulating at least one undersea pipe resting on the sea bottom, in particular at great depths, for connecting the sea bottom to anchored installations floating on the surface.
The invention relates more particularly to pipes connecting the sea bottom to anchored installations floating on the surface.
The technical field of the invention is that of making and assembling insulating systems outside and around pipes for conveying hot effluents from which it is desired to limit losses of heat.
The invention applies more particularly to developing oil fields in deep seas, that is to say offshore oil installations where surface equipment is generally situated on floating structures while the wellheads are at the bottom of the sea. The pipes concerned by the present invention are more particularly risers, i.e. pipes providing a bottom-to-surface connection by rising towards the surface, however the invention also applies to pipes connecting wellheads to said riser pipes.
Present developments in deep seas are generally performed in depths of water that can be as great as 1500 meters (m). Future developments are anticipated in depths of water of as much as 3000 m to 4000 m, and even more.
2. Discussion of Related Art
The main application of the invention is thermally insulating underwater, sub-sea, or immersed pipes or ducts, and more particularly those at great depths, more than 300 m, serving to convey hot petroleum products which will lead to difficulties if they cool excessively, whether under normal production conditions or in the event of production being stopped.
In that type of application, numerous problems arise if the temperature of the petroleum products decreases by a significant amount compared with their production temperature, which production temperature is generally in the range 60° C. to 80° C., or even higher, while the temperature of the surrounding water, particularly at great depths, can be well below 10° C., and can be as little as 4° C. If the petroleum products cool to below a certain temperature T1, which depends on the quality of the products concerned, where the temperature T1 generally lies in the range 20° C. to 60° C., for example, then the following are observed:                a great increase in viscosity, which reduces the flow rate in the pipe;        a precipitation of dissolved paraffin which then increases the viscosity of the product, and by being deposited can reduce the effective inside diameter of the pipe;        flocculation of asphaltenes, leading to the same problems; and        the sudden, compact, and massive formation of gas hydrates which precipitate at high pressure and low temperature, thus suddenly obstructing the pipe.        
Paraffins and asphaltenes remain stuck to the wall which must then be cleaned by scraping the inside of the pipe; in contrast hydrates are even more difficult and sometimes even impossible to resorb.
One of the functions of thermally insulating such pipes is thus to slow down the cooling of the petroleum effluent conveyed so that its temperature does not drop below T1, e.g. 40° C. on reaching the surface, for a production temperature at the inlet of the pipe of T2=60° C. to 80° C., not only under steady production conditions, but also in the event of the rate of production decreasing or even stopping, in order to ensure that the temperature of the effluent does not drop too far below the temperature T1, e.g. below 30° C., in order to limit the above-mentioned problems, or at least to ensure that they remain reversible.
With the installation of single pipes or of bundles of pipes, it is generally preferred to prefabricate the pipes on land in unit lengths of 250 m to 500 m, which lengths are then towed offshore by a tug. For a tower type bottom-to-surface connection, the length of the pipe generally constitutes 50% to 95% the depth of the water, i.e. it can be as much as 2400 m for a depth of 2500 m. While it is being built on land, the first unit length is pulled out to sea and the following length is connected to its end, the tug keeping the assembly under traction throughout the end-to-end joining stage which can last for several hours, or even several days. Once the entire pipe or bundle of pipes is in the water, it is towed to the site, generally below the surface in a substantially horizontal configuration, where it is then “up-ended”, i.e. tilted vertically so as to reach the vertical position, after which it is put in its final position.
A device for thermally insulating at least one undersea pipe is known that comprises an insulating outer covering surrounding the pipe, and an outer protective case, said outer case performing two functions:                firstly preventing damage which could arise during manufacture and towing, and also during laying, particularly in shallow zones, where towing can under some circumstances take place over distances of several hundred kilometers; for this purpose, the materials used are quite strong, such as steel, a thermoplastic or thermosetting compound, or indeed a composite material; and        secondly creating a leakproof confinement around the insulating system; this confinement is necessary with insulating outer coverings constituted by materials that can be subject to migration, or indeed that include fluid compounds.        
In depths of 2000 m, hydrostatic pressure is about 200 bars, i.e. 20 megapascals (MPa), which implies that the set of pipes and their covering of insulating material must be capable of withstanding not only such pressures without damage during pressurization and depressurization of the pipe in which the hot fluid flows, but must also be capable of withstanding temperature cycles that lead to changes in the volumes of the various components, and thus to positive or negative pressures that can lead to partial or total destruction of the case, either by exceeding acceptable stresses, or by implosion of the outer case (pressure variants leading to negative internal pressures).
Document WO 00/40886 discloses an insulating device in which a solid-liquid insulating phase-change material is used having a latent heat of fusion with a phase change that takes place at a temperature T0 that is higher than the temperature T1 at which the petroleum flowing inside the pipe becomes too viscous, where the temperature T1 generally lies in the range 20° C. to 60° C. and is lower than the temperature T2 of the crude oil penetrating into the pipe.
In the event of production stopping, a phase-change material (PCM) makes it possible to conserve the fluid that would normally be flowing inside the inner pipe at a temperature that is high enough to avoid paraffins or hydrates forming in the petroleum product.
Thus, in the event of production stopping, the crude oil ceases to flow and remains in position within the pipe, and the loss of heat to the external environment, generally at 4° C. in very great depths, takes place to the detriment of the PCM, the crude oil continuing to remain at a temperature that is greater than or substantially equal to the phase change temperature of said PCM.
Throughout the solidification or crystallization of the PCM, the temperature of the PCM remains substantially constant and equal to T0, e.g. 36° C., and thus the inner pipe containing the crude oil remains at a temperature that is greater than or substantially equal to the temperature (T0) of the PCM, i.e. 36° C., thus preventing paraffins or hydrates forming in the crude oil.
Said insulating phase-change material is preferably selected for its low thermal conductivity, and in particular conductivity of less than 0.5 watts per meter per degree Celsius (W/m/K).
Said PCM insulating material is selected in particular from materials constituted by at least 90% chemical compounds selected from alkanes, in particular having a hydrocarbon chain of at least 10 carbon atoms, or indeed optionally hydrated salts, glycols, bitumens, tars, waxes, and other fatty materials that are solid at ambient temperature, such as tallow, margarine, or fatty alcohols and acids, and preferably the incompressible material is constituted by paraffin having a hydrocarbon chain of at least 14 carbon atoms.
The phase-change materials described in the past generally present significant change in volume on changing state, this change in volume possibly being as much as 20% for paraffins. The outer protective case must be capable of accommodating such variations in volume without damage.
That is why, according to WO 00/40886, the insulating phase-change material is confined within a leakproof and deformable case that is thus capable of following the expansion and the contraction of the various components under the influence of all of the surrounding parameters, and in particular internal and external temperatures. The pipe is thus confined within a semirigid or flexible thermoplastic case, in particular one made of polyethylene or polypropylene, and one that is circularly shaped, for example, with any increase or reduction in its inside volume due to temperature variations being comparable to breathing and being absorbed by the flexibility of the case which is constituted, for example, by a thermoplastic material presenting a high elastic limit. However, in order to withstand mechanical stresses, it is preferable to use a case that is semirigid, being made of a strong material such as steel or a composite material, for example a compound based on a binder such as epoxy resin and organic or inorganic fibers such as glass fibers or carbon fibers, but under such circumstances the case is given an oval or flattened shape with or without reentrant portions so as to give it a section that for given perimeter is less than that of the corresponding circle. Thus, the “breathing” of the bundle will lead, in the event of an increase and a decrease in volume, respectively to the case being made rounder and to the case being made flatter. Under such circumstances, the case and bundle assembly is referred to as a “flat bundle” in contrast to a circular case.
In WO 00/40886, the PCM is absorbed within an absorbent matrix, and it occupies all of the space that exists between the pipe and said outer case with which it always remains in contact, said case being deformable.
While an insulating device of that invention is being made, which preferably takes place on land, the space between the pipe(s) and the outer case is filled with the PCM while in the liquid state, i.e. while hot. Nevertheless, there is a risk during filling with said PCM that said PCM might solidify locally, thus preventing the volume from being filled completely. Under such circumstances, empty zones or gas pockets are created that are harmful firstly to the insulating effect during future operation of the installation, and secondly and above all to overall strength since there is a risk of the case collapsing locally when the pipe is installed in great depths, i.e. when it is subjected to very high hydrostatic pressures. These problems are easily overcome on short lengths of pipe, for example 6 m or 12 m, but they are much more difficult to avoid over significant lengths, for example more than 100 m.
In order to overcome those drawbacks, techniques have been developed in the prior art that are based on insulating devices comprising an insulating material constituted by a gel that presents a high degree of insulation. Gelling presents the advantage of avoiding convection phenomena within the insulating mass. In addition, the gel is generally obtained by physical, chemical, or physico-chemical reactions between various components, thus enabling the gel to be injected in liquid form immediately after its components have been mixed together, it being possible to fill the case completely before bulk gelling begins in significant manner.
Embodiments have thus been proposed in which the insulating PCM is formulated in the form of particles or microcapsules of said PCM that are uniformly dispersed within a matrix of a primary insulating material, in particular an insulating gel in order to make it easier to occupy the entire space between the pipe and the outer case.
Nevertheless, that technique presents the drawback of the quantity of PCM surrounding the pipe being necessarily reduced since it is distributed discontinuously around the pipe.
In addition, the inventors have found that it is only the PCM that is close to the hot pipe that can accumulate heat on liquefying, since the PCM close to the outer case is generally at the temperature of the sea bottom, i.e. 4° C., and thus does not contribute to the process of accumulating heat, i.e. it remains permanently in the solid or crystallized state. This fraction of the PCM close to the outer case is thus ineffective and useless and can even be harmful if the PCM used presents high intrinsic conductivity, as is the case for metallic salts.
Prior embodiments have also been described for applications in which the pipe rests horizontally on the sea bottom. However certain problems then arise for bottom-to-surface connections.
With a bottom-to-surface connection, for example the vertical portion of a tower, or indeed a catenary section connecting the top of the tower to a support on the surface, or also with pipes resting on a deep slope on the sea bottom, the external pressure varies along the pipe, decreasing on rising towards the surface. With insulating materials that are in paste or fluid form, such as PCMs, the material presents density that is less than that of sea water, generally having a relative density lying in the range 0.8 to 0.85, so a pressure differential between the inside and the outside will vary along said pipe, increasing on approaching the surface. Thus, greatest deformation occurs in the portions that present the greatest pressure differential, thereby leading to important transfers of fluid parallel to the longitudinal axis of said pipe. In addition, such transfers are amplified by the “breathing” phenomena due to temperature variations, as described above.
A “flat bundle” is sensitive to pressure variations due to slopes: higher pressure lower down, lower pressure higher up, and the towing stage is critical since the length of the bundle can be as much as several kilometers, and the bundle is never accurately horizontal, giving rise to significant pressure variations during said towing, and above all during the up-ending operation for a bottom-to-surface connection.
When the bundle is in the vertical position or on the sea bottom on a significant slope, the pressure differential created by the low density of the insulating material, associated with the variation in volume created by the thermal expansion of the insulating material, leads to movements in the insulating material that the outer case must be capable of withstanding. It is desirable to avoid particles moving parallel to the axis of the bundle, i.e. migration of insulating material between two remote zones of the bundle, since that runs the risk of destroying the physical structure proper of the insulating material.
In order to ensure that the bundle behaves well throughout its lifetime, it is desirable for it not to contain any residual gas. With an insulating mixture that is pasty or semifluid, any pocket of gas that results from the manufacturing process will have repercussions, firstly during transport since once the bundle is being towed at significant depth, the ambient pressure will compress the residual gas, which runs the risk of significantly reducing its buoyancy and can lead to situations that are dangerous not only for the equipment but also for personnel; and secondly, while it is being put into a vertical position, any pockets of compressed gas will tend to come together towards the top of the bundle, thus running the risk of creating a significant length of pipe that does not have any insulating compound.